Darkfall by Dean R Koontz

This has to be his best novel to date. It's suspence filled from start to finish and with superb characterization.
This terrifying story takes place over the course of a single day. The police are investigating a series of brutal murders in which the victims are all involved in organised crime, but all is not what it seems and soon the police are facing a far more formidable opponent.
Seriously scary stuff...